As is well known, a ship or other freely floating structure at sea, without propulsion, and subject to the action of swell, performs motion that is complex, and that can be resolved into six motions: three rotary motions; and three linear motions. In a system of orthogonal axes X, Y, and Z, these six motions comprise: roll which is alternating rotation about the X axis; pitching which is alternating rotation about the Y axis; yaw which is alternating rotation about the Z axis; surge which is alternating translation along the X axis; sway which is alternating translation along the Y axis; and heave which is alternating translation along the Z axis.
When two offshore structures are placed side by side, while one of the structures is floating and the other is either fixed or floating, it is sometimes necessary to limit the motions of the two offshore structures relative to each other. This applies, for example, when it is necessary to stabilize a barge carrying a deck which is to be placed on a support structure and fixed thereto. In the context of the present description, the term "deck" is used for any type of platform superstructure installed at sea. The deck conventionally comprises a plurality of vertical tubular legs made of steel or of concrete or partly of steel and partly of concrete, and which are placed on and fixed to a support structure. The term "support structure" is used to designate any type of infrastructure, commonly referred to as a "jacket" in this technical field and designed to support the deck of the offshore platform. In service, the support structure may be completely or partially immersed, and it may optionally stand on a sea bed. Conventionally, the support structure has the same number of legs as the deck. The legs of the support structure are generally vertical or substantially vertical or else they are vertical in part and inclined in part relative to the vertical. In addition, in the context of the present invention, the term "barge" is used to designate any ballastable floating vehicle capable of transporting the deck of an offshore platform.
The deck and the support structure of an offshore platform are conventionally prefabricated separately on land or in a dry dock or in a graving dock, and they are subsequently taken and/or towed separately to a site at sea where they are then assembled together. The assembly site may be the site at which the platform is to be used, or it may be some other site selected for its sufficient depth of water and for sea conditions that are relatively calm. To place the deck on the support structure, the barge carrying the deck is brought between the legs of the support structure. In order to ensure that the deck-placing operation takes place under good conditions when the sea is subject to swell, it may be necessary to restrict the motion of the barge relative to the support structure. The load-bearing surfaces of the deck legs and the load-bearing surfaces of the receiving portions at the top ends of the support structure legs are of limited size, and their sizes may sometimes be smaller than the amplitude of the horizontal motion of the barge. Conventionally, yaw, roll, and sway motions of the barge are limited by guides, optionally including shock absorbers, disposed between the barge and the legs of the support structure. In addition, various known systems have already been proposed to limit surge movements of the barge relative to the support structure. A first known system consists in using anchor lines and/or hawsers connecting the barge to the support structure. However, experience shows that such a known system is inadequate for limiting surge motion. A second known system consists in using guiding pins, i.e. pegs mounted vertically that are movable along the legs of the deck and that are capable of being engaged in cylindrical sockets that are rigidly fixed to the outsides of the legs of the support structure. Here again, experience shows that such a system is fragile, and above all that it is unsuitable for stopping a barge which is subjected to surge motion. A third known system is described, for example, in the article entitled "Offshore installation of an integrated deck onto a preinstalled jacket" by G. J. White et al., OTC 5260, Offshore Technology Conference, 18th Annual Conference at Houston, Tex. May 5-8, 1986, page 322, righthand column and FIG. 4. In the third known system, each leg of the deck (or at least some of them) contains a kind of plunging piston capable of being engaged in a centering tube with an inlet cone provided at the top ends of the corresponding legs of the support structure. A radial shock absorber constituted by a sleeve of elastomer material and referred to as "radial elastomer" is disposed in the annular space between the centering tube and the leg-forming tube of the support structure. The difference between the second and third above-described known systems lies essentially in the presence of the radial elastomer which provides an improvement with respect to fragility, but which constitutes a drawback with respect to preventing motion, specifically because the radial elastomer is compressible. In addition, when the surge motion is large, that third known system becomes too fragile.